Allergic airway inflammation is a hallmark of asthma, a disease that is a significant cause of morbidity in the US. Group 2 innate lymphoid cells (ILC2) produce cytokines that drive the initial phase of allergic airway inflammation. ILC2 are activated by cytokines such as thymic stromal lymphopoietin (TSLP) that are produced in response to airway challenge with Alternaria alternata, a widespread fungal aeroallergen that has been linked to severe asthma exacerbations. To date, there are no known negative regulators of lung ILC2 that are approved by the FDA. However, our novel preliminary data reveals that endogenous PGI2 critically inhibits both airway TSLP expression and the lung expression of IL-5 and IL-13, cytokines expressed by lung ILC2, following intratracheal (IT) challenge of mice with the allergen extract of Alternaria alternata. Additional preliminary data suggests that exogenous PGI2 inhibits innate immune cell expression of TSLP in both mouse and human cells. These preliminary data lead us to propose the hypothesis that PGI2 negatively regulates the expression and signaling of TSLP in response to Alternaria airway challenge and, as a result, inhibits the development and function of lung ILC2. This proposal will determine how PGI2 regulates the activation of host innate immune cells and signaling pathways to the microbial antigens in Alternaria alternata that lead to allergic inflammatory responses. The proposed studies are paradigm shifting in that they will determine the role of PGI2 in blocking IL-5 and IL-13 expression by lung ILC2 and the signaling pathways responsible for this inhibition. These proposed studies are clinically relevant in that we will define potential mechanisms by which PGI2, which is currently FDA approved for the treatment of pulmonary hypertension, may be effective in the treatment of allergic airway inflammatory diseases such as asthma. In addition, we will define how dietary supplementation of ?-3 fatty acid, which reduces endogenous PGI2, regulates the early innate allergic immune response to Alternaria alternata challenge. Such diets have been recommended as asthma therapy, but our preliminary data leads us to hypothesize that ?-3 fatty acid dietary supplementation increases airway TSLP expression and lung ILC2 cell cytokine secretion, thus exacerbating allergic airway inflammation. The proposed studies will advance the field by defining mechanisms that negatively regulate the early innate immune response to the protease containing allergens such as Alternaria alternata.